Rechargeable lithium-ion batteries are attractive energy storage systems for portable electronics and hybrid-electric vehicles because of their high energy density and rate capability. However, they generally suffer degradation mechanisms that limit their useful life. These degradation mechanisms can be classified as power fade (an increase in internal resistance of the battery) and capacity fade (a decrease in useable capacity). Capacity fade, in turn, can be divided into (i) degradation or loss of the active material that serves as a host to the lithium ions in the two working electrodes and (ii) loss of charge due to side reactions at one or both of the electrodes. Christensen, J. and J. Newman, “Effect of Anode Film Resistance on the Charge/Discharge Capacity of a Lithium-ion Battery”, Journal of the Electrochemical Society, 150 (2003) A1416; Christensen, J. and J. Newman, “Cyclable Lithium and Capacity Loss in Li-Ion Cells”, Journal of the Electrochemical Society, 152 (2005) A818.
A typical Li-ion cell 10, as shown in FIG. 1, contains a negative electrode 20, a positive electrode 22, and a separator region 24 between the negative and positive electrodes 20/22. Both electrodes 20/22 contain active materials 26 and 28, respectively, into which lithium can be inserted, inert materials 36, and a current collector 38/40, respectively. The active materials 26/28 are also referred to as lithium-insertion materials. The separator 24 contains an electrolyte with a lithium cation, and serves as a physical barrier between the electrodes 20/22 such that the electrodes 20/22 are not electronically connected within the cell 10.
Typically, during charging, there is generation of electrons at the positive electrode 22 and consumption of an equal amount of electrons at the negative electrode 20, and these electrons are transferred via an external circuit 30. In the ideal operation of the cell 10, these electrons are generated at the positive electrode 22 because there is extraction of lithium ions from the active material 28 of the positive electrode 22, and the electrons are consumed at the negative electrode 20 because there is insertion of lithium ions into the active material 26 of the negative electrode 20. During discharging, the exact opposite reactions occur.
The main charge-transfer reactions that occur at the two electrodes 20/22 during charge, which results in Li+ moving in the direction of arrow 32, are:LiP→Li++e−+P (at the positive electrode 22) andLi++e−+N→LiN (at the negative electrode 20)wherein P represents the positive electrode material 28 and N the negative electrode material 26. Accordingly, LiP and LiN are the positive electrode materials and negative electrode materials, respectively, intercalated with lithium. For discharging, these reactions proceed in the opposite direction with Li+ moving in the direction of arrow 34.
The charge/discharge cycle for an ideal cell is represented in FIGS. 2A-2E. As shown in the figures, lithium (represented by shading) starts in the positive electrode in the discharged state of the cell (FIG. 2A). During charge (FIG. 2B), lithium is transferred to the negative electrode. At full charge, all of the lithium is transferred to the negative electrode (FIG. 2C). During the subsequent discharge (FIG. 2D), the opposite reactions occur, and all of the lithium is transferred back to the positive electrode at full discharge (FIG. 2E). In the ideal operation of the cell, there are no other charge-transfer reactions, besides the main reactions.
Side reactions have been defined as those charge-transfer reactions that occur other than the insertion or extraction of lithium ions into or out of the active material, with common examples including decomposition of the solvent or formation of the solid electrolyte interphase (SEI) at the negative electrode as reported by Arora, P., R. E. White, and M Doyle, “Capacity Fade Mechanisms and Side Reactions in Lithium-ion Batteries”, Journal of the Electrochemical Society, 145 (1998) 3647, and Aurbach, D., “The Role of Surface Films on Electrodes in Li-ion Batteries”, in Advances in Lithium-Ion Batteries, W. A. van Schalkwijk and B. Scrosati, Eds. Academic/Plenem Publishers: New York, 2002; p 7. For non-ideal cells, some charge can be consumed via a side reaction. This results in a permanent capacity loss if the side reaction is not fully reversible. In contrast, the main reactions as described above with respect to FIGS. 2A-2E are typically fully reversible.
FIGS. 3A-3E depict an example in which an irreversible side reaction occurs at the negative electrode 20 during charge, consuming electrons that ideally should be consumed by the main reaction. FIG. 3A represents the initial discharged state of the cell. FIG. 3B represents the cell during charge and FIG. 3C represents the cell after the cell is fully charged. FIG. 3D represents the cell during discharge and FIG. 3E represents the cell after the cell is fully discharged. In FIG. 3B, “S” is a generic reactant that could represent the solvent, an anion, or a contaminant. The product S− may be soluble in the electrolyte, or can form a solid precipitate with the lithium cation. Because this reaction is irreversible in this example, the reverse reaction does not occur during discharge (FIG. 3D), and the charge cannot be transferred back to the positive electrode 22.
The small box 40 below the negative-electrode box 20 thus represents charge that is consumed via the side reaction. It is shaded after the cell is charged to show that some of the charge has been consumed irreversibly (FIG. 3C). However, the total area of the shaded regions in all of the boxes remains constant because charge is conserved. While the example depicted in FIGS. 3A-3E present a completely irreversible reaction, some side reactions may be somewhat reversible, in which case a fraction of the charge consumed by the side reaction can be returned to the positive electrode.
The capacity of the cell is proportional to the number of electrons that are reversibly transferred from one electrode to the other via the external circuit. Thus, as seen from the example in FIGS. 3A-3E, the cell's capacity is reduced because of side reactions.
Some effort has been made to ameliorate the reduced capacity which results from undesired side reactions. U.S. Pat. No. 6,025,093 issued to Herr in 1998, discloses a system wherein cells have been designed to compensate for first-cycle lithium loss during SEI formation. As noted above, SEI is a side reaction.
U.S. Pat. No. 6,335,115, issued to Meissner in 2002 describes the use of an auxiliary lithium electrode that compensates for lithium loss throughout the life of the cell. In the '115 patent, two means of isolating the auxiliary electrode from the working electrodes are disclosed. One such isolation means is ionic isolation and the second isolation means is an electronic isolation. Ionic isolation involves an orientation of the battery in which the lithium-ion containing electrolyte contacts the two working electrodes, but not the auxiliary electrode. The lithium auxiliary electrode is presumably always in electronic contact with one of the working electrodes, but replenishment of lithium to the depleted working electrode does not occur until the cell is reoriented such that the electrolyte is in contact with both the working electrode and the auxiliary electrode.
The ionic isolation approach has some limitations. For example, in a lithium-ion battery the battery would have to be designed such that the electrolyte does not completely fill the pores of the separator and working electrodes. However, the porous separator would naturally act as a wick that transports the electrolyte to the region of the separator that contacts the auxiliary electrode. Even residual electrolyte on the pores of this region of the separator would allow transport of lithium from the auxiliary electrode to the working electrode. Lithium transfer would continue until the potentials of the working and auxiliary electrodes equilibrated. Excessive lithium transfer, beyond the point of capacity balance between the two working electrodes, would result in reduction of the cell's capacity as reported by Christensen, J. and J. Newman, “Effect of Anode Film Resistance on the Charge/Discharge Capacity of a Lithium-ion Battery”, Journal of the Electrochemical Society, 150 (2003) A1416, and Christensen, J. and J. Newman, “Cyclable Lithium and Capacity Loss in Li-Ion Cells”, Journal of the Electrochemical Society, 152 (2005) A818.
Moreover, shorting of the auxiliary-electrode-working-electrode circuit via imperfect ionic isolation would lead to rapid transfer of lithium to the working electrode and possible deposition of lithium on the electrode surface. Such lithium deposition can pose a safety risk and/or degrade the cell because the lithium metal reacts rapidly and exothermically with the organic solvent used in the electrolyte as reported by Arora, P., M. Doyle, and R. E. White, “Mathematical Modeling of the Lithium Deposition Overcharge Reaction in Lithium-ion Batteries Using Carbon-based Negative Electrodes”, Journal of the Electrochemical Society, 146 (1999) 3543.
Even if it were possible to maintain ionic isolation of the auxiliary electrode until lithium transfer is required, the cell design disclosed in the '115 patent would require additional electrode and separator material that is unutilized. Moreover, the orientation of the cell in FIG. 1 of the '115 patent is such that the two working electrodes are not in ionic contact, and therefore, lithium transport between the two electrodes is impossible in this orientation.
Even if the foregoing shortcomings are addressed, a system which relies upon reorientation of the battery significantly reduces the number of potential applications. For example, battery-powered devices such as power tools may be used in any orientation, meaning that the auxiliary-electrode-working-electrode circuit could be closed unintentionally during standard operation of the battery. Hence, the device disclosed in the '115 patent appears to be limited to applications that have a fixed orientation.
Another approach disclosed in U.S. Pat. No. 7,726,975, issued to Christensen et al. in June 2010, involves the use of an auxiliary lithium electrode that can be electronically connected to or isolated from one or more of the working electrodes. The system in the '975 patent circumvents the issues raised above by relying upon electronic, rather than ionic, isolation of the lithium reservoir electrode (LRE) from the working electrodes. The '115 patent also discloses such electronic isolation. However, the lithium auxiliary electrode proposed in the '115 patent is placed “between the positive and negative electrodes.” Such placement would reduce the uniformity of current distribution, and therefore the rate capability of the cell, when transferring lithium from one working electrode to the other. The approach in the '975 patent avoids this problem by placing the LRE outside the current path between the two working electrodes.
Approaches that rely upon an auxiliary lithium electrode typically suffer from the problem of large length scales associated with the distance of the auxiliary electrode from the working electrodes. A typical length scale between two working electrodes, which are pressed together on either side of a porous separator, is on the order of 100 microns, while the distance from an auxiliary lithium electrode to the farthest region of each working electrode can be on the order of 1 cm or more, even when the auxiliary lithium electrode is sandwiched between the two working electrodes. This is because the auxiliary electrode may not span the entire separator, as this would block ionic transport from one working electrode to the other. Moreover, dendrites could easily form and short one or more working electrode with the auxiliary electrode in this case.